Dual Channel Memory Query

[RufusKing]

Looking into memory and just confused myself on the matter.

I have a dual channel MoBo at the moment, and one stick of dual channel DDR400...

A friend of mine has a dual channel MoBo but it only runs at full speed with two sticks of RAM in.

Whats the deal with dual chanel these days? Is it all pretty much dual channel? If someone could clear the matter it would make my memory shopping a lot easier!

-Leonard

[mike_w]

I'm not entirely sure what you're asking. If you want to know what RAM is dual channel, I believe that you just have to have the same size RAM in both channels, and it will work as dual channel.

You don't need to have exactly the same RAM in both channels, but the disadvantage of running two different RAM sticks is that if one is slower, it could slow down the other stick. By having two sticks that are the same, their performance is going to be pretty much the same, so they won't slow down the other stick that much.

[nichomach]

OK, dual channel is dependant upon either whether the chipset supports it (in the case of AthlonXP and P4 rigs) or whether the processor supports it (in the case of Athlon64 systems). If you have a dual-channel capable motherboard but only have one stick of RAM, it is running in single channel mode. Your memory is not in itself dual-channel; if you had another identical stick of DDR400 in a slot on the other channel of your memory controller, then your motherboard would run it in dual-channel mode.

Example: I have an Abit NF7-S v1 motherboard with an AthlonXP. This has a dual channel memory controller. It has 3 DIMM slots. If I stick a 1GB DIMM in slot 1, it will only have memory present on one channel, so will run in single channel mode. If I stick another module in slot 2, that slot runs off the same channel as slot 1, so it'll STILL only run in single channel mode. If I stick the second DIMM in slot 3, that's on the second channel, so with slots 1&3 populated it will run in dual-channel mode. The same situation (with regards to different slots being on different memory controllers and the dual-channel capability being provided by the motherboard chipset) largely obtains with P4 boards as well. Note that the memory in itself is not "dual channel". It's the motherboard that runs the two DIMMs in dual channel mode.

If you look at the Athlon64 situation, the memory controller is actually integrated onto the processor die; Socket 754 processors integrate a single channel memory controller, Socket 939 processors integrate a dual-channel memory controller. Consequently, regardless of what the memory is labelled as, if you bung it in a S754 machine it will always run on a single channel. By contrast, if you stick a single DIMM in an S939 system, that will run in single channel mode, but if you stick two DIMMs in the machine and make sure the slots they inhabit are on different channels the system can run in dual-channel mode.

There's really no such thing as "dual-channel" RAM - there's RAM which has been tested as being stable in dual-channel mode, but that's not the same thing at all.

Does this help any?

[StormPC]

Exactly. DC kits are nothing more than memory that has been paired up because they either were tested as a pair in a DC system and qualified at a certain speed or they came from the same production run. It is likely that a DC kit will perform better when overclocking but it is not a sure thing by any means.

[johnnr892]

is there any advantage really of dual-channel preformance wise. Will dual-channel work on the Sis gigabyte socket A board? ga7s48-l i think.

[Stoo]

about 5% max iirc..

[Thorsson]

But that's 5% on overall performance.

If you have one stick already matching will be difficult. In practise you are probably going to be held back by your original stick.

[Agent]

They dont have to match. Use the search as this has been covered extensivly by several of us before.
It will help if they are matched when overclocking though (as they come from the same batch, they will almost always overclock the same), but its not required.
Same size also helps so it can all be addressed in 128bits, if they are different sizes, the smallest *2 is the maximum amount of memory that can be addressed in 128bit, while the rest is done as 64bit iirc.

[Thorsson]

Mmm yes, I should have said that the hold-back was mainly in OC'ing.

[StormPC]

is there any advantage really of dual-channel preformance wise. Will dual-channel work on the Sis gigabyte socket A board? ga7s48-l i think.

Running DC can be a large difference depending on the platform. It's not quite as important for socket A as it is for the A64, but it is most important for P4 systems as they really need the bandwidth most.